Conventionally, a known technique for a combustion system having an internal combustion engine is multistage injection to inject a fuel into a combustion chamber for multiple times in one combustion cycle. For example, in Patent Literature 1, an injection timing and the like are controlled for post injection among the multistage injection, thereby to enable to increase an unburned fuel in an exhaust gas and to raise an exhaust gas temperature. In the above configuration, a DPF (diesel particulate filter) being a fine particle collecting device is installed in an exhaust passage of the internal combustion engine, and in order to promote combustion of a PM deposited in the DPF, an injection timing of the post injection is set according to a cetane number of the fuel. For example, as the cetane number of the fuel becomes higher, a post injection timing is retarded further, and a post injection amount is set to a smaller value.